Display Zoom is a feature available only on iPhone 6 and iPhone 6 Plus. It allows you to set the display to a zoom mode that shows larger text and controls. The trade off is the loss of real screen estate. In the normal Standard mode, the home screen is displaying six rows of app icons. You get only five rows of app icons when in zoom mode.
iPhone 6 has a native screen pixels of 750 x 1334. When you select the Zoom mode, the display is scaled to a dimension 640 x 1136, the same as the iPhone 5 and 5s. On the iPhone 6 Plus, the zoom mode will scale down the screen to the same 750 x 1334 resolution as the iPhone 6.
If you pixel peep and scrutinize close enough, you will find images to appear slightly blurry and text not as sharp when in zoom mode. Standard mode is the default. If you must use the zoom mode, you can change it at Settings > Display & Brightness > Display Zoom.
Tap on the “Zoomed” and then the “Set” button on top right to change the Display Zoom mode.